The present application relates to an apparatus for continuously forming raised marks on plastic cable jackets which is disposed in the immediate vicinity of the screw extruder producing the cable jacket.
Because of the ever increasing number of different cables laid together in cable ditches or cable chutes it is necessary to provide these cables with marks for easier identification. These marks, e.g. marks of the owner or manufacturer, but also length or generic marks such as marks for telephone or power cables, should clearly stand out against the cable jacket even when viewed from a distance or in badly lighted cable ducts, and should be abrasion-resistant. They should be clearly visible not only on new cables but also on cables which had been in use for a protracted period of time, e.g., buried in the ground.
It is known in the art to obtain such marks by embossing the cable jacket while it is still soft immediately after its production. It is obvious that, because of the cable jacket being mostly made of black or dark plastic, the embossed marks contrast badly with their background and, therefore, are badly visible. To this must be added that, in order for the marks to be visible at all, very heavy embossing is necessary which, however, results in the cable jacket being deformed and its thickness being reduced. To offset these disadvantages, the cable jacket would have to be overly thick, which, however, would have great economic disadvantages because of the considerably higher material expenditure.
A method and an apparatus are known (German Published Application No. 1,951,597) which avoid the aforementioned disadvantages. In the known method, powder of a material equal or similar to that of the cable jacket is applied onto the hot cable jacket through a stencil provided with apertures in accordance with the marks to be formed. Through the residual heat of the cable jacket, the powder is then sintered on the spot into a raised mark which simultaneously fuses with the cable jacket.
The known method and the known apparatus have proved suitable in practice. However, they can only be used to form areal marks such as generic marks, firm-symbols or the like, but do not permit continuous length marks to be applied onto the cable jacket in equal quality and with justifiable expense.